Aluminum is a metallic material of light weight, good thermal conductivity, and relatively high resistance to corrosion in a neutral atmosphere. Thus, it has recently come into widespread popular usage in the form of structural members for chemical equipment and vessels, for example. It is known, however, that when aluminum structural members are used in heat exchangers and liquid storage tanks which are directly exposed to sea water or industrial water (hereinafter generally referred to as "water"), they often develop pitting or grain boundary corrosion, i.e., the phenomenon of uneven corrosion, attributable to a so-called electrochemical cause. Such pitting or grain boundary corrosion of aluminum articles in contact with water is one form of electrochemical phenomenon which is caused by a potential difference between the article and water. As measures for the protection against said electrochemical corrosion of aluminum materials used in environments exposed to water as described above, application of an anodic oxide coating on the surface of such aluminum materials and/or application of a film of paint to such surface have been accepted in actual practice and do prevent such corrosion at least to some extent. When coating of oxide and films of paint alone are relied upon, however, they cannot be expected to produce sufficient protection against corrosion over a long period because these coatings and film have potential faults of their own and the coating eventually undergoes degradation and the film peels off the substrate during prolonged service.
As a technique for the protection of metallic materials immersed in water against electrochemical corrosion, there has been heretofor known the so-called sacrificial anode method which involves attaching to a metallic material requiring protection another metallic material exhibiting a baser natural potential in water than the metallic material requiring protection thereby reducing the potential of the metallic material to be protected in water at all times below the pitting potential. Alternatively, there has been known the so-called cathodic protection method which involves causing a feeble anticorrosion current from an external power source to flow between the metallic material and an opposite electrode immersed in water, thereby keeping down the potential of the metallic material in water at all times below the pitting potential. These methods are widely used for the protection of steel materials against corrosion and are affording desirable results. Alternatively, a weak current from an external power source has been caused to flow between the article and an opposite electrode immersed in the water.
The present inventors made various studies in search for a method capable of protecting either aluminum materials having an anode oxide coating or a film of paint applied to the surface thereof or bare aluminum materials, immersed in water, against pitting or grain boundary corrosion by the application of the aforementioned sacrific anode or cathodic protection method. In all the tests, however, mere application of the conventional sacrifice anode method to such aluminum materials failed to afford the same satisfactory protection against corrosion as had been obtained for steel materials. The reason for this failure is that unlike steel, aluminum is a so-called amphoteric metal which dissolves in both acids and alkalis.
In has been known that for a given aluminum material to remain stable in water for a long time without substantially undergoing corrosion, the natural potential of the aluminum material in water should be maintained in a narrow range from about 0.3 V to 0.4 V below the pitting potential up to the pitting potential, although this range is slightly variable dependent upon the kind of alloy components used in the aluminum material or upon the nature of the environment in which the aluminum alloy is immersed in water. To ensure protection of the aluminum material against corrosion by the use of known methods, therefore, it is necessary that the cathodic potential of the aluminum article to be protected against corrosion should be controlled throughout the entire volume or mass of the aluminum material at all times so as to be retained within the aforementioned range of stable potential as much as possible. To ensure protection of the aluminum material against corrosion by the use of the sacrifice anode, therefore, it is necessary that the cathode potential of the aluminum material subjected to protection against corrosion should be controlled throughout the entire volume of the aluminum material so as to be retained within the aforementioned range of stable potential as much as possible. When the sacrifical electrode is formed of a metal which has potential relatively close to the natural potential of aluminum in water, the portion of the aluminum article which is in the vicinity of the sacrificial anode can be controlled at a proper potential owing to the cathode current flowing between the aluminum material and the sacrificial anode. In contrast, the portion of the aluminum article which is remote from the sacrificial anode cannot be given adequate control of potential because the flow of the cathode current is lowered, by the electrical resistance offered from water. Thus, this remote portion of the aluminum article inevitably suffers pitting or grain boundary corrosion. When the sacrificial anode is made of a metal possessing sufficiently baser natural potential than the aluminum so as to permit control of potential even in the portion of the aluminum material remotely separated from the sacrificial anode, the portion of the aluminum material close to the sacrificial anode is subjected to excessive potential which tends to induce the phenomenon of alkali corrosion due to so-called excessive anticorrosion. This is so with the application of the cathodic protection method wherein an external power source is used. That is, when the voltage of the external power source is controlled so as to maintain the cathode potential at the portion in the vicinity of the opposite electrode of the aluminum material in a proper range, the potential at the portion remote from the opposite electrode is insufficiently repressed. On the other hand, when it is contemplated to repress sufficiently the potential at the portion remote from the opposite electrode of the aluminum material, the potential at the portion in the vicinity of the opposite electrode is excessively repressed. Such excessive repression of the potential tends to cause dissolution, i.e. alkali corrosion, of the aluminum material. As described above, when the conventional sacrificial anode method or cathodic protection method with use of the external power source is relied on, it is difficult to effect control of the cathode potential of the entire volume of the aluminum material so that the potential may remain in the stable range. This difficulty has notably restricted the application of the sacrifice anode method to aluminum materials.